Transit of Earth and Moon (and Phobos) from Mars on 2084-November-10

Yu-Ann Wang

2017-February-12

Recommended places to be

  • Orbit
  • Earth-Moon-Transit: Gale Crater (MSL/Curiosity Rover), 02:00 to 13:05 UT
  • Triple transit: 126.6E, 28.3N (north of EZ12 Hebrus Valles), 08:08 to 08:10 UT

In orbit

On 10th of November 2084, the orbits of Earth and Mars will align such that when viewed from Mars, both Earth and the Moon will transit across the disk of the Sun. Here is a representation of what a viewer may expect to see.

As always, please remember to wear solar-filters for a safe viewing experience. Do NOT look at the sun directly with naked eyes.

Ephemeris data courtesy of the wonderful NASA JPL’s HORIZONS On-Line Ephemeris System.

It goes without saying that the night-side of the Earth will be facing Mars during the transit. At the start of the Earth transit at 02:03 UT, Europe, Africa and the Atlantic will be Mars-facing. By the end at of the Earth transit at 10:28 UT, the Americas and the Pacific will be Mars-facing. More of the Earth’s northern hemisphere will be visible as the southern hemisphere inclines towards the Sun in November as in this 2016-11-20 HiRISE image of Earth-Moon from Mars. Resolving the familiar night-time city lights of human civilisation from Mars while the Earth is silhouetted against the solar disk is likely an interesting photography challenge.

The public science outreach potential of this event will be immense. In-person viewing will have the most impact (ref. the overview effect). Real-time web-casts (albeit with a delay of 3:58 minutes due to the finite speed of light) from space-telescopes or other satellites in orbit around Mars will increase the reach and message spread considerably. (Ref. SOA coverage of the Venus Transit 2012)

The lead time available till 2084 offers ample lead time for interested parties to mount an expedition and put hardware and communications in place.

From the surface

2084 November 10th falls in the Month 11 of the Martian year, which is the middle of the Martian Dust Storm Season. Now, should the weather hold with clear skies, it will be possible to view the transit from the Martian surface.

A number of viewing sites are considered. These are either historical landing sites, or locations where future infrastructure are considered. Future proposed sites are from the Mars 2020 landing site short list, and the proposed Exploration Zones (EZ) from the 2015 HLS2 workshop. Elevation details, where not provided, are estimated from MOLA imagery.

The best sites will be ones where

  • the sun will be high above the horizon (high elevation) for better viewing
  • both transits can be seen
  • if not, Earth is prioritized for public outreach reasons

Also, ideally, any such site should have provisions for a decent observatory / telescope set-up, which implies supporting infrastructure. An EZ can provide this: targetted to land in the 2030s (or ‘within the next 50 years’), an EZ will make an ideal base for further settlement-based activities.

The figure below shows the distribution of landing sites and how visible the twin transit will be from their respective locations. (Open image on new tab for a higher resolution view.)

Of the historical and proposed Mars2020 and HLS2 sites, only two locations offer views of both the Earth and Moon transit: Curiosity/MSL/Gale Crater, and Polarlander. Between the two, Gale Crater offers a higher solar elevation, making it the preferred choice. Here is a view from the Mars24 software showing the sunlit face of Mars at 07:57UT. The yellow dot shows the sub-solar point.0757 UT

In addition, Spirit/MERA/Gusev Crater and surrounds is in view for the full Earth transit. It has a more favourable solar elevation for Earth ingress than Gale Crater. However, the Sun and the Moon sets during the lunar transit.

Phobos

The Moon is often an interfering body in Earth-based astronomy. It interferes by ‘polluting’ the night-sky with moonlight when it is above the horizon, interfering with the observation of non-Lunar objects. Phobos, the larger moon of Mars, appears as an interfering body in the sky during the transit time frame. This, however, offers an intriguing opportunity for a triple-transit. Due to its ‘potatoroid’ shape, Phobos has an angular diameter anywhere between 660 to 830 arc-seconds when viewed from the surface of Mars (over the period analysed), which is not quite large enough to fully eclipse the disk of the Sun, which has an angular diameter of 1308.9 arc-seconds. Curiosity captured such a transit in 2013. The triple-transits will be observable at points directly beneath Phobos’s shadow as it travels across the sky.

The chart below shows ground locations where Phobos will occult the Earth, with their respective centres coming as close as less than 0.02 degrees (or 72 arc-seconds) apart during the occultation. These locations are sampled at 2 degree longitude intervals, adjusted (broadly) for terrain height. The elevation at which the solar disk et. al. are above the horizon at the time of the occultation varies significantly: the lower the latitude, the higher the elevation.

Here is a list of ground locations near historical or proposed landing sites where Phobos will come closer than five degrees to the Sun and the Earth during this transit. Azimuth and Elevation (degrees) refer to Earth’s Azimuth and Elevation in the sky. Minimum TOI refers to the closest separation in between Phobos and Earth, and what time it does so. Curiosity is referenced for reference.

##   n                   site Azimuth(d) Elevation(d) Min. TOI(d)  When
##   1    EZ11a Erebus Montes   243.1825       8.2432       1.040 08:29
##   2      EZ2 Phlegra Dorsa   227.3198      21.7041       2.341 08:25
##   3 EZ18 Amazonis Planitia   239.5749       7.2634       2.956 08:27
##   4     EZ12 Hebrus Valles   172.7765      54.7872       3.418 08:09
##   5       EZ10 Nili Fossae   117.3509      21.7209       4.729 07:50
##  14            H Curiosity   215.5640      77.1609      16.600 08:14

Unfortunately, none of these locations are in the direct path of Phobos’s shadow.

What if overland travel is possible? Then, of these five EZ

  • EZ11a Erebus Montes offers a shortest drive to a triple transit viewing point
  • EZ12 Hebrus Valles is the staging point to the highest solar elevation viewing point if distance is less of a concern

Let’s take a walk drive. Below show transit views at each of the five EZ.

EZ11a Erebus Montes

With only 1.04 degrees separating Earth and Phobos at closest approach, EZ11a Erebus Montes (192.1E, 39N) is a good candidate from which to search for a triple transit viewing point. Proceeding due south, at (192.1E, 36N), approximately 177km away (which is an significant distance compared to the 100km wide EZ) a partial triple transit (figure left below) is expected. Another 0.5 degree south to (192.1E, 35.5N) (approximately 30km), a full triple transit (figure right below) is expected. However, the Sun will be only 9.6 degrees above the horizon at the time of the transit.

Caveat: As Phobos is not spherical (but a potatoroid), the Phobos disk drawn is an approximation.

Note that Phobos’s transit is complete within one minute. This is due to Phobos’s very short orbital period of 7 hours 39.2 minutes. It moves around Mars faster than Mars itself rotates, which is why it rises in the West and sets in the East. Phobo’s small size and short orbital period result in a high sensitivity of transit visibility to latitude.

There is lower sensitivity to longitude as Phobos’s inclination is only 1 degree to Mars’ equator. The following shows the view at locations 0.5 degrees West (figure left below) and East (figure right below) of (192.1E, 35.5N) respectively.

EZ2 Phlegra Dorsa

The same ‘proceed south’ routine from EZ2 Phlegra Dorsa (172E, 39N) brings the triple transit into view at (172E, 31.25N), approximately 458km (figure below) away from the EZ.

EZ18 Amazonis Planitia

The same ‘proceed south’ routine from EZ18 Amazonis Planitia (188E, 46.16N) brings the triple transit into view at (188E, 34.6N), approximately 684km (figure below) from the EZ.

EZ12 Hebrus Valles

A ‘proceed north’ routine from EZ12 Hebrus Valles (126.6E, 20.1N) brings the triple transit into view at (126.6E, 28.3N), approximately 486km (figure below) from the EZ.

EZ10 Nili Fossae

The same ‘proceed north’ routine from EZ10 Nili Fossae (76.95E, 22.05N) brings the triple transit into view at (76.95E, 35.5N), approximately 796km (figure below).

Next Steps

Getting there. (TBC)

Key References:

  • HORIZONS System: http://ssd.jpl.nasa.gov/?horizons
  • Mars24: https://www.giss.nasa.gov/tools/mars24/
  • HLS2 Workshop Abstracts: https://www.nasa.gov/journeytomars/mars-exploration-zones
  • Mars2020 candidate sites: http://marsnext.jpl.nasa.gov/workshops/wkshp_2017_02.cfm
  • Earth-Mars Date Converter: http://www-mars.lmd.jussieu.fr/mars/time/martian_time.html